Cryogenic engines

ABSTRACT

Slush gas, i.e. a gas or a mixture of gases cooled so that it is partially solid and partially liquid is employed as a drive fluid in a cryogenic engine. A cryogenic engine has a working chamber ( 50 ) connected to an energy source comprising a body of slush gas ( 47 ) via injection apparatus having a housing ( 36 ) which acts as a heat exchanger for causing part of the slush gas entering the injector to boil, so as to enable the gas to be driven under pressure into the working chamber ( 15 ).

FIELD OF THE INVENTION

This invention relates to cryogenic engines and in particular to drive fluids therefor. The term ‘cryogenic engine’ is used to include, but is not limited to, any contrivance for producing mechanical (and/or electrical) power from a cryogenic drive fluid through boiling and expansion. Mechanical power includes output shaft power and thrust power from a jet. Thus, for example, the term engine includes within its scope engines having reciprocating drive members, rotary engines, including turbines, and jet or non-chemical rocket engines.

BACKGROUND TO THE INVENTION

PCT specification WO01/63099 discloses a cryogenic engine employing a liquefied gas (such as liquefied nitrogen) as the drive fluid. Liquefied nitrogen has the disadvantage that, in the event of leakage into an enclosed space, the resulting atmosphere can cause asphyxiation to humans and other mammals. Also, liquefied nitrogen is very difficult to pump at high pressures because of its tendency to vaporise. The invention stems from work done to identify an alternative drive fluid for a cryogenic engine.

SUMMARY OF THE INVENTION

According to one aspect of the invention slush gas is employed as a drive fluid in a cryogenic engine. The term slush gas as used herein means a gas, or a mixture of gases, cooled or compressed so that it is partially solid and partially liquid.

The slush gas may be slush air, which is air cooled (typically to below minus 210° Celsius) so that it is partially solid and partially liquid.

The drive fluid may be supplied to the engine as slush gas. In another example of the invention the drive fluid is a liquid derived from a slush gas by prior melting of the solid constituents of the latter, the slush gas thus still constituting the source of the drive fluid.

The slush air may be pre-treated by removing one or more constituents of air before it is frozen. It is particularly desirable to remove moisture (i.e. water vapour) from the air and such removal may be undertaken by using a known refrigerated drying process according to which the air is refrigerated to condense the water vapour which is then drained off. In addition, or as an alternative, carbon dioxide may be removed, for example by physical separation during compression or chemical absorption, both of which are processes known per se.

Slush air is easier and cheaper to produce than liquid nitrogen and slush air contains more energy than liquid nitrogen. Slush air has an energy density of 1.5 to 1.6 Mega Joules per kilogram, compared with 1.23 for liquid air and 1.1 for liquid nitrogen. Also, slush air is safer and easier to transport since any increase in temperature would in the first instance result in the solid component melting rather than the liquid component boiling, resulting in less build up of pressure and less overall waste. Moreover, any slush air which does boil results in a harmless emission.

The proportion of liquid to solid in the slush air may vary, but the slush air must be sufficiently flowable (or fluid) to be injected into the cylinder or other working chamber of the cryogenic engine. Preferred proportions for liquid: solid are in the range 70:30, most preferably 60:40.

Instead of forming the slush gas by cooling air, the slush gas may be formed by cooling individual gaseous constituents (e.g. nitrogen and oxygen) and then combining the cooled constituents to form the slush gas.

Alternatively, the slush gas may be slush nitrogen which is nitrogen cooled so that it is partially solid and partially liquid.

According to another aspect of the invention a cryogenic engine is made to utilise slush gas as its drive fluid.

The cryogenic engine, to which the slush gas is delivered, may be used to power a vehicle (for example a motor road vehicle) or a stationary engine (for example to generate electricity).

Injection apparatus is preferably used to deliver the slush gas under pressure to the engine cylinder or other working chamber in which the drive fluid (constituted by the slush gas) expands to provide the shaft power.

The invention also includes within its scope a method of generating shaft power utilising slush gas, preferably slush air, as the energy source.

The injection apparatus preferably comprises a housing, and an injection member moveable within the housing in order to displace the drive fluid from a first region of the housing to a second region of the housing, in use the drive fluid, in a slushy condition, being admitted to the first region of the housing and transferred to the second region by movement of the injection member, the second region being at a higher temperature than the first region, causing a small volume of the drive fluid in the second region to boil and thereby inject the drive fluid into the cryogenic engine under pressure.

The injection member is preferably moveable within the housing in a repetitive sequence timed to be in appropriate synchronism with the working cycle of the cryogenic engine, which may follow a two-stroke or a four-stroke cycle. The injection apparatus may be driven by the cryogenic engine or may alternatively be driven by a separate power source, such as an electric motor. On startup, the apparatus may be primed by passing the slush gas through the first region, in order to cool the latter.

The injection member is preferably reciprocatable within the housing, undergoing injection strokes and return strokes in alternate sequence. On an injection stroke, the member may move towards the second region, carrying a volume of drive fluid with it, and in this case the member may make sealing engagement with the housing and have a recess into which the volume of drive fluid is admitted at the first region and from which it is delivered at the second region.

Alternatively, the injection member may move towards the first region on an injection stroke, displacing the drive fluid from the first region to the second region, and in this case the housing is preferably equipped with inlet and outlet valves which open and close in timed manner to admit the drive fluid to the first region at the commencement of an injection stroke and allow egress of the drive fluid before the commencement of the return stroke.

The injection apparatus consumes little power which is conveniently obtained from the shaft power of the associated cryogenic engine.

The invention includes within its scope a body of slush gas adapted for use as a drive fluid (or source thereof), that is as the energy source, for a cryogenic engine.

The body of slush gas may be held in a container, preferably insulated, at atmospheric pressure or above. The container may be a fuel tank of the cryogenic engine which may be a stationary engine or may power a vehicle such as a motor road vehicle, or the container may be a storage tank (fixed or mobile) for refuelling a cryogenic engine. Such a body of slush gas may be viewed as an energy storage battery having the advantages of ready availability of constituent gases, high energy density, capability of storage for extended periods of time, ease of transport and absence of pollution when used.

In the case of a fuel tank for a small vehicle such as a scooter, golf buggie, moped, motorbike, or lawn mower, or of a small household generator, the container may have a capacity of up to 100 litres (which also corresponds to the volume of the body of slush gas therein). A container for use as a fuel tank of a motor car will have a capacity of at least 100 litres, preferably 250-300 litres, the body of slush gas being of a corresponding initial volume. If the body of slush gas is to be used to propel a bus the initial volume of the body, and hence the capacity of its container, is preferably at least 1000 litres.

A body of slush gas could be used as a means of storing energy derived from off peak power produced by a power station (for example a nuclear power station), in which case the volume of the body could be of the order of millions of litres and could be stored in large underwater or underground storage tanks.

BRIEF DESCRIPTION OF THE DRAWINGS

Four embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1 to 3 illustrate the first embodiment at three different stages in an operative cycle,

FIGS. 4 to 7 illustrate the second embodiment at four different stages in its operative cycle,

FIG. 8 illustrates the third embodiment in conjunction with part of cryogenic engine,

FIG. 9 is a sectional view on the line IX-IX of FIG. 8, and

FIGS. 10 and 11 correspond to FIGS. 8 and 9, but show the fourth embodiment.

Two examples of injection apparatus for injecting slush air into a cryogenic engine will now be described, by way of example, with reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE DRAWINGS

Each example of injection apparatus is designed to inject a charge of slush air into the working chamber of a cryogenic engine which is preferably as disclosed in PCT specification WO 01/63099 the disclosure of which is incorporated herein by reference. The slush air is a mixture of liquefied air and solidified air, in the ratio of 60:40 liquid to solid so that the mixture is sufficiently fluid to be injected. In the working chamber of the engine, the slush air receives energy from a heat exchange liquid and expands to produce the engine shaft power.

The injection apparatus of FIGS. 1 to 3 comprises an injection member in the form of a plunger 4 which reciprocates within a cylindrical housing 6 and makes sealing engagement therewith. The housing 6 has a first region 8 (which is at a sufficiently low temperature for the slush air) and a second region 10 which is at ambient temperature, typically between 10° C. and 20° C. The wall of the housing has a portion of increased diameter forming an annular inlet chamber, and the plunger has a portion of decreased diameter, forming a waisted region defining an annular volume.

The plunger 4 undergoes alternate injection and return strokes in order to take slush air 2 from a source thereof at low pressure and deliver it under pressure into the working chamber of a cryogenic engine.

To achieve this, the source of low pressure slush air is placed in communication with the annular inlet chamber. At the start of an injection stroke (FIG. 1), the annular inlet chamber and the waisted region in the plunger are in communication so the annular volume fills with slush air 2 at low pressure. The plunger 4 then moves downwardly within the housing 6 to undertake an injection stroke (FIG. 2), the plunger making sealing engagement with the housing wall so that the volume of slush air 2 within the chamber is carried with the plunger 4 from the region 8 towards the region 10. On reaching the region 10, a small amount of the transferred slush air boils as it is subjected to the higher temperature in the region 10, creating a source of high pressure which, at the end of the injection stroke where the chamber is no longer uncovered by the housing wall, drives the dose of slush air into the chamber of the cryogenic engine (FIG. 3).

Referring to FIGS. 4 to 7, the second embodiment of injection apparatus comprises a plunger 20 reciprocatable, with clearance, in a cylindrical housing 22 having an inlet valve 24 for controlling admission of low pressure slush air from a source thereof, and an outlet valve 26 for controlling flow of high pressure slush air from the housing 22 to the working chamber of a cryogenic engine. The valve 26 has a valve stein 30 which passes through a passage in the plunger 20.

At the commencement of an injection stroke (FIG. 4) the inlet valve 24 is open and the outlet valve 26 is closed, slush air at low pressure being admitted to a first region 32 within the housing at low temperature. The inlet valve 24 then closes and the plunger 20 commences an injection stroke, moving upwardly within the housing 22 towards the low temperature region 32 (FIG. 5). During the injection stroke of the plunger 20, slush air is transferred by displacement from the low temperature region 32 to a second region 34 at ambient temperature. At the end of the injection stroke (FIG. 6) substantially all the slush air has been transferred and a small volume of nitrogen boils as a result of the higher temperature in the region 34. The resulting high pressure opens the outlet valve 26 and causes the slush air to be injected under high pressure into the working chamber of the cryogenic engine.

In each embodiment, the flow of slush air through the apparatus will maintain the first (low temperature) region at the required low temperature. The second (higher temperature) region will be maintained at the required higher temperature by drawing heat from the cylinder or casing of the cryogenic engine, or from being in contact with the heat exchange liquid. The apparatus may be driven by the cryogenic engine (eg. from the cam shaft thereof) or may be driven from a separate electric motor. The amount of slush air entering the apparatus can be controlled (eg. by a valve) or by controlling the speed of the pump associated with the cryogenic engine.

The injection apparatus shown in FIGS. 8 and 9 has a generally cylindrical housing 36 within which extends an array of twelve heat exchange pipes 38 through which passes a heat exchange liquid 40 such as ethylene glycol. At an inlet region of the housing, an inlet valve 42 controls the admission of slush air 44 which is supplied to the injection apparatus by a supply pipe 46 communicating with an insulated pressurised storage tank 47 holding a supply of the slush air at about minus 200° C. For the sake of clarity, the tank is shown schematically and at a reduced scale. At an outlet region of the housing, an outlet valve 48 controls the delivery of slush air now under pressure, to the cylinder 50 of a two-stroke cryogenic engine having a piston 52 reciprocatable within the cylinder 50.

The inlet valve 42 is formed by a movable valve member having an elongated stem 54 terminating at its lower end in a valve head 56 co-operating with a valve seating 58 on the lower end of a cylindrical guide 60. The valve member stein 54 slides within the guide 60 and is sealed with respect to the inner surface of the guide 60 by a circumferential seal 62. The supply pipe 46 communicates with the lower end of the guide 60, just above the valve seating.

Similarly, the outlet valve 48 has a moveable valve member with an elongate stem 64 terminating at its lower end in a valve head 66 co-operating with a valve seating 68 on the lower end of a cylindrical guide 70. The valve member stem 64 slides within the guide 70 and is sealed with respect to the inner surface of the guide 70 by a circumferential seal 72.

Just above the valve seating 68, an outlet pipe 74 communicates with the lower end of the guide 70, leading into the upper end of the cylinder 50. The upper end of the cylinder 50 has two valves, namely a valve 76 for admitting heat exchange liquid 40 to the cylinder 50 and a valve 78 for exhausting heat exchange liquid and drive fluid through an exhaust pipe 80. The cryogenic engine is a two-stroke engine and functions in the manner disclosed in WO01/63099. The injection apparatus and cryogenic engine of FIGS. 8 and 9 operate in the following manner.

Commencing with the piston 52 at top dead centre (as illustrated in FIG. 8) the inlet valve 42 is closed, the outlet valve 48 opens and the two valves 76 and 78 are closed. A charge of drive fluid (i.e. slush air) is forced from the housing 36 through the open outlet valve 48 and into the cylinder 50 where the drive fluid expands (whilst absorbing heat from the heat exchange liquid in the cylinder 50) to cause the piston 52 to undergo a power stroke to drive the crankshaft engine. Towards the end of the power stroke, as the piston 52 approaches bottom dead centre, the exhaust valve 78 opens and the outlet valve 48 remains open until the piston 52 is just beyond bottom dead centre, to reduce the pressure in the housing when the outlet valve closes. During the return stroke of the piston 52 the outlet valve 48 closes and, as soon as possible thereafter, the inlet valve 42 opens. This causes a charge of drive fluid to be admitted to the space, surrounding the pipes, within the housing 36. The heat exchange fluid 40 passing through the pipes 38 transfers thermal energy to the drive fluid, causing a small amount of drive fluid to boil so as to increase the pressure in the housing with the result that when the outlet valve 48 opens at the commencement of the next power stroke the drive fluid is injected into the cylinder under pressure. During the return stroke of the piston 52 the valve 76 opens to admit heat exchange liquid to the cylinders 50.

The described valve timings require the inlet and outlet valves 42 and 48 to undergo operative cycles at the same speed as the cryogenic engine. This places a considerable demand on the inlet valve 42, and to meet this problem the injection apparatus may be duplicated (or replicated any number of times). For example, a pair of injection apparatus, each as shown in FIGS. 8 and 9, may be arranged beside one another so as to supply a single cryogenic engine, each of the two apparatus then operating at half the speed which would be necessary if the engine were supplied by a single apparatus.

The heat exchange liquid supplied to the housing 36 is the same liquid as that supplied to the cylinder 50 through the inlet valve 76. The liquid supplied to the housing 36 is preferably taken, by means of a branched connection, from the main heat exchange liquid supplied to the cylinder, the liquid outlet from the housing 36 being fed back into the return of the heat exchange liquid after this has been exhausted from the cylinder 50. The inlet and outlet valve guides 60 and 70 and the stems 54 and 64 are elongated so that the seals 52, 72 can be located remotely from the low temperature regions at the lower ends of these valves.

In the injection apparatus of FIGS. 10 and 11, parts corresponding to those of FIGS. 8 and 9 bear the same reference numerals. The areas of difference in FIGS. 10 and 11 are: the manner in which the supply pipe 46 continues past the guide 60; the increased spacing of the junction between the pipe 46 and the guide 60 above the lower end of the guide 60; and the formation of the inlet valve member as a spring-loaded check valve member 84 biased towards its closed position in which the check valve member 84 engages with the lower end of the guide 60. The inlet valve 42 is opened by downward movement of the stem 54 which not only traps a volume of drive fluid in the lower length of the guide 60 but also then forces open the check valve member 84 so as to press this volume of drive fluid into the housing 36.

The supply pipe 46 is continued beyond the guide 60, leading the slush air back to the storage tank under the influence of a small re-circulating pump, preferably located in the storage tank. This low speed circulation reduces the tendency for bubbles to form in the slush air.

After entering the housing 36, the slush air receives heat from the heat exchanger so that a small portion boils, driving the slush air through the outlet valve 48 and into the cylinder 50, in the manner described with reference to FIGS. 8 and 9. 

1. The use of slush gas as a drive fluid, or as a source of drive fluid, in a cryogenic engine.
 2. The use of slush gas in accordance with claim 1, wherein the slush gas is slush air.
 3. The use of slush air in accordance with claim 2, wherein the slush air is pre-treated by removing one or more constituents of air before it is liquefied and partially frozen.
 4. The use of slush air in accordance with claim 3, in which said constituents, which are removed, are water vapour and carbon dioxide.
 5. The use of slush gas in accordance with claim 1, in which the proportion of liquid to solid in the slush gas are in the range 70:30.
 6. The use of slush gas in accordance with claim 5, in which the proportions are in the range 60:40.
 7. A method of operating a cryogenic engine, the method comprising the step of supplying to the engine a cryogenic drive fluid comprising, or derived from, slush gas.
 8. A method according to claim 7, in which the slush gas is injected directly into a working chamber of the engine.
 9. A method according to claim 8, in which said injection is achieved by admitting a charge of the drive fluid into an inlet region of an injector housing via an inlet valve, closing said inlet valve, transferring heat to the fluid as the latter passes towards an outlet region of the housing, causing a small volume of the drive fluid to boil, and thereby injecting the fluid into the engine under pressure.
 10. A method according to claim 8, in which the fluid is injected into the engine through an outlet valve of the injector.
 11. A method according to claim 10, further comprising operating the valves in a timed relationship such that when the inlet valve opens to admit a charge of drive fluid the outlet valve is closed, after which the inlet valve closes, the pressure of drive fluid within the housing rises and the outlet valve opens for the delivery of the drive fluid under pressure.
 12. A method according to any of claim 8, in which the cryogenic engine, to which the slush gas is delivered, is used to power a vehicle or a stationary engine.
 13. A cryogenic engine for utilising slush gas as a drive fluid, the engine having injection apparatus delivering slush gas under pressure to the engine cylinder or other working chamber in which the drive fluid expands to provide the shaft power produced by the engine.
 14. An engine according to claim 13, in which the injection apparatus comprises a housing, an inlet valve for controlling the admission of the drive fluid to an inlet region of the housing and an outlet valve for controlling the delivery of the drive fluid from an outlet region of the housing, the housing being such that heat is transferred to the drive fluid in its passage from the inlet region to the outlet region, causing a small volume of the drive fluid in the housing to boil and thereby inject the drive fluid through the outlet valve and into the cryogenic engine under pressure.
 15. An engine according to claim 14, in which the inlet valve and the outlet valve operate in timed relationship such that when the inlet valve opens to admit a charge of drive fluid the outlet valve is closed, after which the inlet valve closes, the pressure of the drive fluid within the housing rises and the outlet valve opens for the delivery of the drive fluid under pressure.
 16. An engine according to claim 14, in which the housing is formed as a heat exchanger for the passage of a heat exchange liquid in order to transfer heat from the heat exchange liquid to the drive fluid.
 17. An engine according to claim 16, in which the heat exchanger is constituted by a plurality of pipes, which extend through the housing and through which the heat exchange liquid is passed.
 18. An engine according to claim 17, in which the pipes extend from the inlet region to the outlet region of the housing, the drive fluid occupying the spaces which is within the housing and which surrounds the pipes.
 19. An engine according to claim 13, in which the injection apparatus comprises a housing, and an injection member moveable within the housing in order to displace the drive fluid from a first region of the housing to a second region of the housing, in use the drive fluid, in a liquefied condition, being admitted to the first region of the housing and transferred to the second region by movement of the injection member, the second region being at a higher temperature than the first region, causing a small volume of the drive fluid in the second region to boil and thereby inject the drive fluid into the cryogenic engine under pressure.
 20. An engine according to claim 19, in which the injection member is moveable within the housing in a repetitive sequence timed to be in appropriate synchronism with the working cycle of the cryogenic engine, which may follow a two-stroke or a four-stroke cycle.
 21. An engine according to claim 20, in which the injection member is reciprocatable within the housing, undergoing injection strokes and return strokes in alternate sequence.
 22. An engine according to claim 21, in which on an injection stroke, the member moves towards the second region, carrying a volume of drive fluid with it, the member making sealing engagement with the housing and having a recess into which a volume of drive fluid is admitted at the first region and from which it is delivered at the second region.
 23. An engine according to claim 21, in which the injection member moves towards the first region on an injection stroke, displacing the drive fluid from the first region to the second region, the housing being equipped with inlet and outlet valves which open and close in timed manner to admit the drive fluid to the first region at the commencement of an injection stroke and allow egress of the drive fluid before the commencement of the return stroke.
 24. An energy source for a cryogenic engine, the energy source comprising a body of slush gas adapted for use as a drive fluid (or source thereof) for the cryogenic engine.
 25. An energy source according to claim 24, in which the body of slush gas is held in a container at atmospheric pressure or above.
 26. An energy source according to claim 25, in which the container is a fuel tank of the cryogenic engine.
 27. An energy source according to claim 25, in which the container is a storage tank (fixed or mobile) for refuelling a cryogenic engine.
 28. An energy source according to claim 26, in which the container has a capacity of up to 100 litres, which also corresponds to the maximum volume of the body of slush gas therein.
 29. An energy source according to claim 26, in which the capacity of the container, and hence the maximum volume of the body is 250-300 litres.
 30. An energy source according to claim 26, in which the capacity of the container and hence the initial volume of the body, is at least 1000 litres.
 31. An energy source according to claim 24, in which the body of slush gas is used as a means of storing energy derived from off peak power produced by a power station, the volume of the body being of the order of millions of litres.
 32. A cryogenic engine and an energy source connected thereto, the energy source comprising slush gas for use as a drive fluid, or source thereof, for the engine.
 33. A cryogenic engine and energy source in accordance with claim 32, wherein the cryogenic engine has injection apparatus apparatus delivering slush gas under pressure to the engine cylinder or other working chamber in which the drive fluid expands to provide the shaft power produced by the engine. 